If you mean that an entity should not advance to the next location unless it is available:
Try using Node Lists when routing entities to the next location. Lists automatically block the entity from moving forward if the destination server is not available.
1. Create a node list in Definitions>Lists
2. Set the input and output buffers to 0 on your servers
3. On the output nodes, set Entity destination type to "Select From List" and choose the node list.
If you mean you have a group entities that are processing at different servers and none in that group can advance until all are processed:
Use a Wait step and event. After each entity finishes, trigger the wait step to wait for an event. After the last entity finishes, fire the Event. You will probably also use a Decide step to separate the non-last entities (wait for event) from the last entity (fire event).